1. Field of the Invention
The present invention relates to a vacuum interrupter, and more particularly concerns an electrode configuration thereof.
2. Description of the Related Art
A vacuum interrupter includes a cylindrical evacuated envelope closed at both ends thereof, made of an insulation material such as glass or ceramic and evacuated inside at high vacuum; electrode rods respectively arranged on both closed ends of the evacuated envelope; spiral-ring-shaped coil electrodes provided on the opposing ends of the electrode rods; disc shaped contacts; and reinforcement members for reinforcing the contacts, wherein one of the electrode rods is moved in the axial direction so as to bring both contacts (namely, a stationary contact and a movable contact) into and out of contact with each other, thereby turning electric current on and off.
In the vacuum interrupter described above, a magnetic field in the axial direction is generated by the electric current and an electric arc inevitably produced between the contacts during interrupting operation is trapped within the diameter of the contacts, whereby thermoelectron and metal vapor produced by the arc converge within the contacts.
For example, as shown in Japanese Unexamined Patent Application Publication No. 11-16456, FIG. 7 is a partially sectional side view showing a configuration of an essential part of a conventional vacuum interrupter, and FIG. 8 is an exploded perspective view showing one electrode assembly of the conventional vacuum interrupter. Referring to Figures, reference numerals 1, 1 denote two electrode assemblies arranged in opposed relationship, each of the electrode assemblies 1 includes an electrode rod 11 placed on one closed end of an evacuated envelope not shown; a spiral-ring-shaped coil electrode 12 having a ring shaped holding part 12a, a part of which is inserted by an opposing end of the electrode rods 11; a pin shaped reinforcement member 13 having an end inserted into the holding part 12a; and a disc shaped contact C whose rear face is in contact with the coil electrode 12 and the reinforcement member 13, wherein respective axle centers of the electrode assemblies coincide with each other.
The coil electrode 12 has a plurality of arm parts 12b, which are integrally formed in radial directions from the ring shaped holding part 12a into which an end 11a of the electrode rod 11 is inserted. Each end of the respective arm parts 12b extends in the same direction along the circumference to form arcuate part 12c. In addition, each end of the respective arcuate parts 12c protrudes in the direction opposing to the contact C to form a joint part 12d, and thus the respective joint parts 12d are brought into contact with the associated contact C.
In a vacuum interrupter for interrupting a heavy-current, an electric arc is produced between the contacts C, C during interrupting operation and currents due to the arc do not flow in the reinforcement members 13, 13 which are made of high resistance material such as stainless steel, but flow to the electrode rods 11, 11 through the coil electrodes 12, 12 which are made of conductive material. The currents flow in the contacts C, C in radial directions. After that, each of the respective currents flows from the joint parts 12d to the coil electrode 12. The current passes the arcuate parts 12c, the arm parts 12b, the holding part 12a, and the electrode rod 11 in order. Consequently, four fan-shaped current paths are formed, and vertical magnetic fields (magnetic fields in the axial direction) of the electrode assemblies 1, 1 are generated in these fan-shaped current paths by the known right-handed screw rule.
As described above, by effectively generating the vertical magnetic fields, the arc which otherwise would be diffused in the evacuated envelope converges within the diameter of the contacts C, C. The arc is not concentrated on the contacts C, C, but diffused on the opposed entire surfaces of the contacts C, C, thereby significantly improving interruption characteristic.
Contact resistance between the stationary contact and the movable contact of the vacuum interrupter is largely affected by a state of contact between the contacts and has large irregularities. The reason is that even if a contact shape is plane, the state of contact between the contacts during closing operation of the vacuum interrupter represents point contact from a micro view-point, because the contact resistance varies in accordance with contact positions, contact areas, or the number of contact points. Particularly in the coil electrode, i.e., in the vacuum interrupter with the vertical magnetic field electrode structure, a current flowing from the electrode rod flows to the contact through the coil electrode, thereby tending to increase contact resistance.
Further, when a large current is applied to the vacuum interrupter, an electromagnetic repulsive force acts between the contacts of the vacuum interrupter. If an external applying force exerted on the vacuum interrupter, when applied current, is smaller than the electromagnetic repulsive force, an electric arc may be produced. The electromagnetic repulsive force is also largely affected by contact positions, contact areas, or the number of contact points between the contacts. If the vacuum interrupter produces an electric arc, chances of deposition of the contacts are high. There arises a problem in that a large external applying force is required so as not to produce an electric arc, and consequently configuration of interrupters or switchgear etc. becomes large.